Austenitic stainless steel

ABSTRACT

An austenitic stainless steel and a thermal reactor constructed therefrom consisting essentially of, in weight percent, carbon 0.15 max., manganese 3.0 max., phosphorus 0.04 max., sulfur 0.04 max., silicon 1.0 max., nickel 22 to 32, chromium 20 to 30, nitrogen 0.10 max., boron 0.01 max., and the balance iron.

United States Patent 1191 Ornitz et a1.

[ AUSTENITIC STAINLESS STEEL [75] Inventors: Martin N. Ornitz, Pittsburgh;

Arthur Moskowitz, New Brighton Twsp., Beaver County; Jerome P. Bressanelli, Pittsburgh, all of Pa.

[73] Assignee: Crucible 1nc., Pittsburgh, Pa. [22] Filed: Mar. 13, 1972 [21] Appl. No.: 234,305

[ Jan.8, 1974 Primary Examinerl-lyland Bizo't Attrney--Clair X. Mullen, Jr.

[5 7] ABSTRACT H C] H 75/128 F 75/l28 N An austenitic stainless steel and a thermal reactor con- 151 Int. Cl. C22C 39/28 fumed herefmm Consisting essentially of, in Weight 581 Field of Search 75/128 N 128 c Percent, Carbon (1-15 manganese P phorus 0.04 max., sulfur 0.04 max., silicon 1.0 max., [56] References Cited nickel 22 to 32, chromium to 30, nitrogen 0.10

UNITED STATES PATENTS max., boron 0.01 max., and the balance iron. 2,121,391 6/1938 Arness /128 N 3 Claims, 4 Drawing Figures f e l /900/- IP23 BASE 6 MANGANESE CONTENT/} Pmmaom 8 m4 SHEET-1 0F 3 FIG:

3 Q a H F H I! H 1 F 0 w w 0 6 9 w I Y H m M lll 0 0 0 0 0 0 0 0 0 0 0 5 0 5 w m w m 5 5 4 w w 3 2 @SEE $5 SEQSE EC 35s 5 mmawsmkm $61 I saw Etw 3% Emma BASE 6 MANGANESE CONTENT (98/ PATENTEI] JAN 8 I974 SHEET 2 OF 3 FIG 2 0 H 2 W wi w H m 2. s 7 M 1\ m 0 w w 0 m w w SILICON CONTENT FIG. 3

NICKEL CONTENT (7o) 1 AUSTENITIC STAINLESS STEEL In view of recent efforts with respect to pollution abatement various steps have been taken to reduce the emissions, particularly of unburnt hydrocarbons from stainless steel having chromium and nickel within the ranges of 20 to 30 percent and 22 to 32 percent, respectively, with a boron additive of up to 0.01 percent, while controlling manganese and silicon. More specifithe exhaust of motor vehicles. For this purpose it is 5 cally, the desired result is achieved by an alloy within known to use a thermal reactor which is essentially an the ranges set forth on Table l. afterburner in that unburnt components of the exhaust TABLE I gases such as hydrocarbons and carbon monoxide are further oxidized in the afterburner to nontoxic gas, e.g.

Element Broad Range Preferred Ran e T ical CO Thls further combustlon takes place in the therg yp mal reactor which is a separate combustion chamber s 6 mm 83 12 5 anganese max. max. outslde the engine of the vehicle. In order to achieve Phosphorus 0.04 m 004 m 0030 sausfactory oxidation of the unburnt hydrocarbons and Sulfur 0.04.max. 0.04 max. 0.015 the like it is necessary that the reactor operate at temg "32 2 -3 peratures on the order of l,500 to l,900 F. Typically, 15 chromium 23 23 25:0 AISI Type 330 has been used for the purpose but in B 0J0 Q05 fth h. h k l t t k l Boron 0.0! max. 0.00l/0.005 0002 view 0 e 1g me e con en e.g. percent n1c e "on y Hal BaL the materlal costs for the reactor are extremely hlgh. Alternately AISI Type 310 has been proposed but it does not have sufficient resistance to distortion during 20 Table 11 lists alloy Compositions melted and tested in high temperature service. establishing the above composition limits.

i TABLE ll COMPOSITIONS OF EXPERIMENTAL ALLOYS Alloy Identification C Mn P S Si Ni Cr N B Remarks 25 25 0.05 1.38 0.034 0.014 0.53 23.84 24.58 0.08 0.002 T-330 0.04 1.68 0.011 0.015 1.14 34.90 19.00 0.05 0.002 'r-310 0.05 1.52 0.020 0.012 0.31 19.52 24.59 0.05 0.001 IP16 0.069 1.05 1.11 14.24 20.04 0.06 0.002 also 1.10 Al 1F22 0.077 6.02 1.17 14.30 19.38 0.06 0.002 also 1.70 Al 11 23 0.075 11.39 1.09 14.30 18.47 0.06 0.002 also 1.70 Al 1F7 0.066 1.18 0.31 19.65 19.74 0.07 11-"9 0.067 1.15 1.06 19.90 19.89 0.06 lFlO 0.064 1.15 2.09 19.90 19.81 0.07 11=5 0.059 1.08 0.81 33.80 19.24 0.06 11% 0.066 1.01 0.56 23.98 19.68 0.07 1H64 0.090 6.24 2.22 13.73 24.58 0.39 1H65 0.093 6.17 2.16 13.67 24.48 0.39 0.002

It is accordingly a primary object of the present invention to provide an austenitic stainless steel particularly adapted for use in the manufacture of thermal reactors having good resistance to deformation at high temperature, as characterized by good creep resistance, oxiation resistance, weldability, all of which are achieved with a relatively low nickel content.

These and other objects of the invention as well as a complete understanding thereof may be obtained from the following description, specific examples and drawings, in which:

FIG. 1 is a graph showing the effect of manganese on the oxidation resistance of alloys of the type of the present invention;

FIG. 2 is a graph showing the effect of silicon with respect to oxidation resistance;

F 10.3 is a graph showing the effect of nickel with respect to thermal expansion; and

FTG. 4 is a graph showing the effect of chromium on .oxidation resistance at high temperatures.

Broadly in the practice of the invention the abovestated object is achieved by providing an austenitic TABLE III Creep Extension at lndicated Stress* Alloy 3000 psi 3500 psi T-3l0 8.8,1U.8,f(92) f( 5).f(99) T-33O 7.7,f(7O),f( 3) l4.0,f(92).fl96) Individual test results. numbers indicate percent elongation in IOOhr. discontinued tests. The letter 1' indicates creep failure and the number in parenthesis denotes time of failure (hrs.).

To determine oxidation resistance the alloys of Table IV were tested at 2,000 F to determine weight gain at various degrees of high temperature exposure.

TABLE IV CYCLIC OXIDATION TESTS* Alloy Weight Gain (mg/in?) After Indicated Time (Hours) at Cyclic oxidation tests conducted as follows:

Test specimens were placed in porcelain crucible and weighed. The crucible containing the specimens was then placed in an air furnace. The test cycle consisted of heating in still air at the desired temperature for 20 hours and then air cooling to room temperature. Crucibles containing the specimens were reweighed and, since all oxide that formed was collected in the crucibles, a net weight gain was recorded. The test cycle was repeated until the desired total exposure time was attained.

As may be seen from the results reported in Table IV the Alloy 2525 in accordance with the present invention exhibited better oxidation resistance than did the conventional alloys.

To determine weldability Alloy 2525, in accordance with the invention, was subjected to TIG welding at speeds of up to 100 inches per minute; with the alloy in accordance with the invention crack-free welds were produced over a wide range of welding conditions and heat inputs. In contrast AISI Type 330 exhibited significant weld cracking under identical conditions.

TABLE V ROOM TEM PERATURE TENSILE PROPERTIES The results as reported in Table V show the superior room and elevated temperature tensile properties of Alloy 2525 in accordance with the present invention in comparison with conventional Types 310 and 330.

To achieve the results as reported above with respect to the desired combination of properties it is necessary that the alloying elements be closely controlled. Specifically carbon must be kept low to avoid sensitization in welding and in cooling after annealing. Accordingly carbon must be restricted to 0.08 percent max. in the preferred composition. Although manganese is a useful addition for deoxidation purposes it may be seen from FIG. 1 that manganese contents above about 2 or 3 percent decreased oxidation resistance during cyclic tests at l,900 and 2,000 F for 200 hours.

Phosphorus and sulfur each should be maintained below about 0.04 percent to ensure good resistance to weld cracking. Also silicon is significant for improving oxidation resistance at high temperatures as may be seen from FIG. 2 showing steels having 20 percent chromium and 20 percent nickel containing varying silicon contents and tested at l,900 and 2,000 F for 200 hours. Although silicon is significant for this purpose it has been found that if silicon is above 0.7 or 1 percent the susceptibility of the alloy to weld cracking increases. Specifically, the 2525 alloy of Table I, but containing 1.19 percent silicon, consistently showed longitudinal weld cracking after TIG welding; whereas, Alloy 2525 of Table I containing 0.53 percent silicon was immune to cracking during identical welding operations.

Nickel is a critical addition for high temperature (creep) strengths and is preferred within the range of 24 to 30 percent; higher or lower nickel levels than claimed result in lower creep strength. Also, as shown in FIG. 3, nickel has a strong effect on the oxidation resistance at high temperature and for this purpose also is preferred within the range of 24 to 30 percent. Chromium is also important for high temperature strength and, as shown in FIG. 4, is critical for oxidation resistance and also for resistance to corrosion in the intended exhaust gas environment within the range of 22 to 28 percent. FIG. 4 reports cyclic tests at 1,900 and 2,000 F for 200 hours. If, however, chromium is above about 30 percent it will undly embrittle the alloy, aside from increasing the cost thereof.

With respect to boron, the addition to the alloy of the invention promotes good creep resistance as indicated for tests on modified Type 309 tested at l,700 F, the results of which are reported in Table VI. On the other hand, boron contents above about 0.005 percent and 0.01 percent are detrimental to sensitization resistance and hot workability.

We claim:

1. An austenitic stainless steel characterized by resistance to distortion at high temperature, and resistance to oxidation and scaling consisting essentially, in weight percent, of carbon 0.08 max., manganese 2.0 max., phosphorus 0.04 max., sulfur 0.04 max., silicon 0.7 max., nickel 24 to 30, chromium 23 to 28, nitrogen 0.08 max., boron 0.001 to 0.005 and the balance iron.

claim 1. 

2. An austenitic stainless steel characterized by resistance to distortion at high temperature, and resistance to oxidation and scaling consisting essentially, in weight percent, of carbon 0.05, manganese 1.70, phosphorus 0.030, sulfur 0.015, silicon 0.50, nickel 25.0, chromium 25.0, nitrogen 0.05, boron 0.002 and the balance iron.
 3. A thermal reactor made from the stainless steel of claim
 1. 